A hybridization-induced charge-transfer state energy arrangement reduces nonradiative energy loss in organic solar cells

Here, we explain why the Energy Gap Law and the energy inversion related to the charge-transfer state have opposite effects on the trend of nonradiative energy loss of organic solar cells. The root is the existing condition of energy inversion. There is indeed a certain probability of energy inversion, but it will eventually be implicit or explicit as determined by the hybridization, which depends on the electron-withdrawing unit of the donor, giving rise to different stacking sites. The triplet-state hybridization leads to an explicit characteristic, while singlet-state hybridization leads to an implicit characteristic.

Tuning Charge-Transfer States by Interface Electric Fields

Intermolecular charge-transfer (CT) states are extended excitons with a charge separation on the nanometer scale. Through absorption and emission processes, they couple to the ground state. This property is employed both in light-emitting and light-absorbing devices. Their conception often relies on donor-acceptor (D-A) interfaces, so-called type-II heterojunctions, which usually generate significant electric fields. Several recent studies claim that these fields alter the energetic configuration of the CT states at the interface, an idea holding prospects like multicolor emission from a single emissive interface or shifting the absorption characteristics of a photodetector. Here, we test this hypothesis and contribute to the discussion by presenting a new model system. Through the fabrication of planar organic p-(i-)n junctions, we generate an ensemble of oriented CT states that allows the systematic assessment of electric field impacts. By increasing the thickness of the intrinsic layer at the D-A interface from 0 to 20 nm and by applying external voltages up to 6 V, we realize two different scenarios that controllably tune the intrinsic and extrinsic electric interface fields. By this, we obtain significant shifts of the CT-state peak emission of about 0.5 eV (170 nm from red to green color) from the same D-A material combination. This effect can be explained in a classical electrostatic picture, as the interface electric field alters the potential energy of the electric CT-state dipole. This study illustrates that CT-state energies can be tuned significantly if their electric dipoles are aligned to the interface electric field.

First-principles design of heavy-atom-free singlet oxygen photosensitizers for photodynamic therapy

In photodynamic therapy (PDT) treatment, heavy-atom-free photosensitizers (PSs) are a great source of singlet oxygen photosensitizer. Reactive oxygen species (ROS) are produced by an energy transfer from the lowest energy triplet excited state to the molecular oxygen of cancer cells. To clarify the photophysical characteristics in the excited states of a few experimentally identified thionated (>C=S) molecules and their oxygenated congeners (>C=O), a quantum chemical study is conducted. This study illustrates the properties of the excited states in oxygen congeners that render them unsuitable for PDT treatment. Concurrently, a hierarchy is presented based on the utility of the lowest-energy triplet excitons of thionated compounds. Their non-radiative decay rates are calculated for reverse-ISC and inter-system crossover (ISC) processes. In addition, the vibronic importance of C=O and C=S bonds is clarified by the computation of the Huang-Rhys factor, effective vibrational mode, and reorganization energy inside the Marcus-Levich-Jörtner system. ROS generation in thionated PSs exceeds their oxygen congeners as kf ≪ kISC, where radiative decay rate is designated as kf. As a result, the current work offers a calculated strategy for analyzing the effectiveness of thionated photosensitizers in PDT.

Atomic-scale visualization of the interlayer Rydberg exciton complex in moiré heterostructures

Excitonic systems, facilitated by optical pumping, electrostatic gating or magnetic field, sustain composite particles with fascinating physics. Although various intriguing excitonic phases have been revealed via global measurements, the atomic-scale accessibility towards excitons has yet to be established. Here, we realize the ground-state interlayer exciton complexes through the intrinsic charge transfer in monolayer YbCl3/graphite heterostructure. Combining scanning tunneling microscope and theoretical calculations, the excitonic in-gap states are directly profiled. The out-of-plane excitonic charge clouds exhibit oscillating Rydberg nodal structure, while their in-plane arrangements are determined by moiré periodicity. Exploiting the tunneling probe to reflect the shape of charge clouds, we reveal the principal quantum number hierarchy of Rydberg series, which points to an excitonic energy-level configuration with unusually large binding energy. Our results demonstrate the feasibility of mapping out the charge clouds of excitons microscopically and pave a brand-new way to directly investigate the nanoscale order of exotic correlated phases.

Molecularly induced order promotes charge separation through delocalized charge-transfer states at donor-acceptor heterojunctions

The energetic landscape at the interface between electron donating and accepting molecular materials favors efficient conversion of intermolecular charge-transfer (CT) states into free charge carriers (FCC) in high-performance organic solar cells. Here, we elucidate how interfacial energetics, charge generation and radiative recombination are affected by molecular arrangement. We experimentally determine the CT dissociation properties of a series of model, small molecule donor-acceptor blends, where the used acceptors (B2PYMPM, B3PYMPM and B4PYMPM) differ only in the nitrogen position of their lateral pyridine rings. We find that the formation of an ordered, face-on molecular packing in B4PYMPM is beneficial to efficient, field-independent charge separation, leading to fill factors above 70% in photovoltaic devices. This is rationalized by a comprehensive computational protocol showing that, compared to the more amorphous and isotropically oriented B2PYMPM, the higher structural order of B4PYMPM molecules leads to more delocalized CT states. Furthermore, we find no correlation between the quantum efficiency of FCC radiative recombination and the bound or unbound nature of the CT states. This work highlights the importance of structural ordering at donor-acceptor interfaces for efficient FCC generation and shows that less bound CT states do not preclude efficient radiative recombination.

Ab Initio Study of Charge Separation Dynamics and Pump-Probe Spectroscopy in the P3HT/PCBM Blend

We develop a bottom-up computational method for excited-state dynamics and time-resolved spectroscopy signals in molecular aggregates, on the basis of ab initio excited-state calculations. As an application, we consider the charge separation dynamics and pump-probe spectroscopy in the amorphous P3HT/PCBM blend. To simulate quantum dynamics and time-resolved spectroscopy, the model Hamiltonian for single-excitation and double-excitation manifolds was derived on the basis of fragment-based excited-state calculations within the GW approximation and the Bethe-Salpeter equation. After elucidating the energetics of the electron-hole separation and examining linear absorption spectrum, we investigated the quantum dynamics of exciton and charge carriers in comparison with the pump-probe transient absorption spectra. In particular, we introduced the pump-probe excited-state absorption (ESA) anisotropy as a spectroscopic signature of charge carrier dynamics after exciton dissociation. We found that the charge separation dynamics can be probed by the pump-probe ESA anisotropy dynamics after charge-transfer excitations. The present study provides the fundamental information for understanding the experimental spectroscopy signals, by elucidating the relationship between the excited states, the exciton and charge carrier dynamics, and time-resolved spectroscopy.

Adsorption Structures Affecting the Electronic Properties and Photoinduced Charge Transfer at Perylene-Based Molecular Interfaces

Perylene-based organic semiconductors are widely used in organic electronic devices. Here, we studied the ultrafast excited state dynamics after optical excitation at interfaces between the electron donor (D) diindenoperylene (DIP) and the electron acceptor (A) dicyano-perylene-bis(dicarboximide) (PDIR-CN2 ) using femtosecond time-resolved second harmonic generation (SHG) in combination with large scale quantum chemical calculations. Thereby, we varied in bilayer structures of DIP and PDIR-CN2 the interfacial molecular geometry. For an interfacial configuration which contains a edge-on geometry but also additional face-on domains an optically induced charge transfer (CT) is observed, which leads to a pronounced increase of the SHG signal intensity due to electric field induced second harmonic generation. The interfacial CT state decays within 7.5±0.7 ps, while the creation of hot CT states leads to a faster decay (5.3±0.2 ps). For the bilayer structures with mainly edge-on geometries interfacial CT formation is suppressed since π-π overlap perpendicular to the interface is missing. Our combined experimental and theoretical study provides important insights into D/A charge transfer properties, which is needed for the understanding of the interfacial photophysics of these molecules.

Spectroscopic Identification of the Charge Transfer State in Thiophene/Fullerene Heterojunctions: Electroabsorption Spectroscopy from GW/BSE Calculations

Creation of charge transfer (CT) states in bulk heterojunction systems such as C60/polymer blends is an important intermediate step in the creation of carriers in organic photovoltaic systems. CT states generally have small oscillator strengths in linear optical absorption spectroscopy owing to limited spatial overlap of electron and hole wave functions in the CT excited state. Electroabsorption spectroscopy (EA) exploits changes in wave function character of CT states in response to static electric fields to enhance detection of CT states via nonlinear optical absorption spectroscopies. A 4 × 4 model Hamiltonian is used to derive splittings of even and odd Frenkel (FR) excited states and changes in wave function character of CT excited states in an external electric field. These are used to explain why FR and CT states yield EA lineshapes which are first and second derivatives of the linear optical absorption spectrum. The model is applied to ammonia-borane molecules and pairs of molecules with large and small B-N separations and CT or FR excited states. EA spectra are obtained from differences in linear optical absorption spectra in the presence or absence of a static electric field and from perturbative sum over states (SOS) configuration interaction singles χ(2) and χ(3) nonlinear susceptibility calculations. Good agreement is found between finite field (FF) and SOS methods at field strengths similar to those used in EA experiments. EA spectra of three C60/oligothiophene complexes are calculated using the SOS method combined with GW/BSE methods. For these C60/oligothiophene complexes, we find several CT states in a narrow energy range in which charge transfer from the thiophene HOMO level to several closely spaced C60 acceptor levels yields an EA signal around 10% of the signal from oligothiophene.

Interface Engineering in Perylene Diimide-Based Organic Photovoltaics with Enhanced Photovoltage

The introduction of nonfullerene acceptors (NFA) facilitated the realization of high-efficiency organic solar cells (OSCs); however, OSCs suffer from relatively large losses in open-circuit voltage (V_OC) as compared to inorganic or perovskite solar cells. Further enhancement in power conversion efficiency requires an increase in V_OC. In this work, we take advantage of the high dipole moment of twisted perylene-diimide (TPDI) as a nonfullerene acceptor (NFA) to enhance the V_OC of OSCs. In multiple bulk heterojunction solar cells incorporating TPDI with three polymer donors (PTB7-Th, PM6 and PBDB-T), we observed a V_OC enhancement by modifying the cathode with a polyethylenimine (PEIE) interlayer. We show that the dipolar interaction between the TPDI NFA and PEIE─enhanced by the general tendency of TPDI to form J-aggregates─plays a crucial role in reducing nonradiative voltage losses under a constant radiative limit of V_OC. This is aided by comparative studies with PM6:Y6 bulk heterojunction solar cells. We hypothesize that incorporating NFAs with significant dipole moments is a feasible approach to improving the V_OC of OSCs.

Fast Recombination of Charge-Transfer State in Organic Photovoltaic Composite of P3HT and Semiconducting Carbon Nanotubes Is the Reason for Its Poor Photovoltaic Performance

Although the photovoltaic performance of the composite of poly-3-hexylthiophene (P3HT) with semiconducting single-walled carbon nanotubes (s-SWCNT) is promising, the short-circuit current density j_SC is much lower than that for typical polymer/fullerene composites. Out-of-phase electron spin echo (ESE) technique with laser excitation of the P3HT/s-SWCNT composite was used to clarify the origin of the poor photogeneration of free charges. The appearance of out-of-phase ESE signal is a solid proof that the charge-transfer state of P3HT⁺/s-SWCNT^- is formed upon photoexcitation and the electron spins of P3HT⁺ and s-SWCNT^- are correlated. No out-of-phase ESE signal was detected in the same experiment with pristine P3HT film. The out-of-phase ESE envelope modulation trace for P3HT/s-SWCNT composite was close to that for the polymer/fullerene photovoltaic composite PCDTBT/PC₇₀BM, which implies a similar distance of initial charge separation in the range 2-4 nm. However, out-of-phase ESE signal decay with delay after laser flash increase for P3HT/s-SWCNT composite was much faster, with a characteristic time of 10 µs at 30 K. This points to the higher geminate recombination rate for the P3HT/s-SWCNT composite, which may be one of the reasons for the relatively poor photovoltaic performance of this system.

Spectroscopic analysis of vibrational coupling in multi-molecular excited states

Multi-molecular excited states accompanied by intra- and inter-molecular geometric relaxation are commonly encountered in optical and electrooptical studies and applications of organic semiconductors as, for example, excimers or charge transfer states. Understanding the dynamics of these states is crucial to improve organic devices such as light emitting diodes and solar cells. Their full microscopic description, however, demands sophisticated tools such as ab initio quantum chemical calculations which come at the expense of high computational costs and are prone to errors by assumptions as well as iterative algorithmic procedures. Hence, the analysis of spectroscopic data is often conducted at a phenomenological level only. Here, we present a toolkit to analyze temperature dependent luminescence data and gain first insights into the relevant microscopic parameters of the molecular system at hand. By means of a Franck-Condon based approach considering a single effective inter-molecular vibrational mode and different potentials for the ground and excited state we are able to explain the luminescence spectra of such multi-molecular states. We demonstrate that by applying certain reasonable simplifications the luminescence of charge transfer states as well as excimers can be satisfactorily reproduced for temperatures ranging from cryogenics to above room temperature. We present a semi-classical and a quantum-mechanical description of our model and, for both cases, demonstrate its applicability by analyzing the temperature dependent luminescence of the amorphous donor-acceptor heterojunction tetraphenyldibenzoperiflanthene:C₆₀ as well as polycrystalline zinc-phthalocyanine to reproduce the luminescence spectra and extract relevant system parameters such as the excimer binding energy.

Picosecond Charge-Transfer-State Dynamics in Wide Band Gap Polymer-Non-Fullerene Small-Molecule Blend Films Investigated via Transient Infrared Spectroscopy

Organic solar cells based on wide band gap polymers and nonfullerene small-molecule acceptors have demonstrated remarkably good device performances. Nevertheless, a thorough understanding of the charge-transfer process in these materials has not been achieved yet. In this study, we use Fano resonance signals caused by the interaction of broad electronic charge carrier absorption and the molecular vibrations of the electron acceptor molecule to monitor the charge-transfer state dynamics. In our time-resolved infrared spectroscopy experiments, we find that in the small-molecule acceptor, they have additional dynamics on the order of a few picoseconds. A change in the solvent used in thin film deposition, leading to different morphologies, influences this time further. We interpret our findings as the dynamics of the charge-transfer state at the interface of the electron donor and the electron- acceptor. The additional mid-infrared transient signal is generated in this state, as both electron and hole polarons can interact with small-molecule acceptor vibrational modes.

Advances in Flexible Organic Photodetectors: Materials and Applications

Future electronics will need to be mechanically flexible and stretchable in order to enable the development of lightweight and conformal applications. In contrast, photodetectors, an integral component of electronic devices, remain rigid, which prevents their integration into everyday life applications. In recent years, significant efforts have been made to overcome the limitations of conventional rigid photodetectors, particularly their low mechanical deformability. One of the most promising routes toward facilitating the fabrication of flexible photodetectors is to replace conventional optoelectronic materials with nanomaterials or organic materials that are intrinsically flexible. Compared with other functional materials, organic polymers and molecules have attracted more attention for photodetection applications due to their excellent photodetection performance, cost-effective solution-fabrication capability, flexible design, and adaptable manufacturing processes. This article comprehensively discusses recent advances in flexible organic photodetectors in terms of optoelectronic, mechanical properties, and hybridization with other material classes. Furthermore, flexible organic photodetector applications in health-monitoring sensors, X-ray detection, and imager devices have been surveyed.

Importance of structural hinderance in performance-stability equilibrium of organic photovoltaics

Power conversion efficiency and long-term stability are two critical metrics for evaluating the commercial potential of organic photovoltaics. Although the field has witnessed a rapid progress of efficiency towards 19%, the intrinsic trade-off between efficiency and stability is still a challenging issue for bulk-heterojunction cells due to the very delicate crystallization dynamics of organic species. Herein, we developed a class of non-fullerene acceptors with varied side groups as an alternative to aliphatic chains. Among them, the acceptors with conjugated side groups show larger side-group torsion and more twisted backbone, however, they can deliver an efficiency as high as 18.3% in xylene-processed cells, which is among the highest values reported for non-halogenated solvent processed cells. Meanwhile, decent thermal/photo stability is realized for these acceptors containing conjugated side groups. Through the investigation of the geometry–performance–stability relationship, we highlight the importance of side-group steric hinderance of acceptors in achieving combined high-performance, stable, and eco-friendly organic photovoltaics.

The intrinsic trade-off between efficiency and stability is a challenging issue for organic photovoltaics. Here, the authors develop non-fullerene acceptors with tailored side-group hindrance and backbone planarity and realise efficiency of 18.3% and good stability for xylene-processed solar cells

A Transparent Electrode Based on Solution-Processed ZnO for Organic Optoelectronic Devices

Achieving high-efficiency indium tin oxide (ITO)-free organic optoelectronic devices requires the development of high-conductivity and high-transparency materials for being used as the front electrode. Herein, sol-gel-grown zinc oxide (ZnO) films with high conductivity (460 S cm⁻¹) and low optical absorption losses in both visible and near-infrared (NIR) spectral regions are realized utilizing the persistent photoinduced doping effect. The origin of the increased conductivity after photo-doping is ascribed to selective trapping of photogenerated holes by oxygen vacancies at the surface of the ZnO film. Then, the conductivity of the sol-gel-grown ZnO is further increased by stacking the ZnO using a newly developed sequential deposition strategy. Finally, the stacked ZnO is used as the cathode to construct ITO-free organic solar cells, photodetectors, and light emitting diodes: The devices based on ZnO outperform those based on ITO, owing to the reduced surface recombination losses at the cathode/active layer interface, and the reduced parasitic absorption losses in the electrodes of the ZnO based devices.

A highly conductive and transparent electrode is essential to achieving a high efficiency in indium tin oxide-free optoelectronic devices. Here, the authors strategically prepare sol-gel-grown zinc oxide films based on photoinduced doping effect and demonstrate enhanced performance of devices.

Molecular Oligothiophene-Fullerene Dyad Reaching Over 5% Efficiency in Single-Material Organic Solar Cells

A novel donor-acceptor dyad, 4, in which the conjugated oligothiophene donor is covalently connected to fullerene PC₇₁ BM by a flexible alkyl ester linker, is synthesized and applied as photoactive layer in solution-processed single-material organic solar cells (SMOSCs). Excellent photovoltaic performance, including a high short-circuit current density (J_SC ) of 13.56 mA cm^-2 , is achieved, leading to a power conversion efficiency of 5.34% in an inverted cell architecture, which is substantially increased compared to other molecular single materials. Furthermore, dyad 4-based SMOSCs display excellent stability maintaining 96% of the initial performance after 750 h (one month) of continuous illumination and operation under simulated AM 1.5G irradiation. These results will strengthen the rational molecular design to further develop SMOSCs for potential industrial application.

Robust ΔSCF calculations with direct energy functional minimization methods and STEP for molecules and materials

The direct energy functional minimization method using the orbital transformation (OT) scheme in the program package CP2K has been employed for Δ self-consistent field (ΔSCF) calculations. The OT method for non-uniform molecular orbitals occupations allows us to apply the ΔSCF method for various kinds of molecules and periodic systems. Vertical excitation energies of heteroaromatic molecules and condensed phase systems, such as solvated ethylene and solvated uracil obeying periodic boundary conditions, are reported using the ΔSCF method. In addition, a Re-phosphate molecule attached to the surface of anatase (TiO₂) has been investigated. Additionally, we have implemented a recently proposed state-targeted energy projection ΔSCF algorithm [K. Carter-Fenk and J. M. Herbert, J. Chem. Theory Comput. 16(8), 5067-5082 (2020)] for diagonalization based SCF in CP2K. It is found that the OT scheme provides a smooth and robust SCF convergence for all investigated excitation energies and (non-)periodic systems.

Probing Charge Generation Efficiency in Thin-Film Solar Cells by Integral-Mode Transient Charge Extraction

The photogeneration of free charges in light-harvesting devices is a multistep process, which can be challenging to probe due to the complexity of contributing energetic states and the competitive character of different driving mechanisms. In this contribution, we advance a technique, integral-mode transient charge extraction (ITCE), to probe these processes in thin-film solar cells. ITCE combines capacitance measurements with the integral-mode time-of-flight method in the low intensity regime of sandwich-type thin-film devices and allows for the sensitive determination of photogenerated charge-carrier densities. We verify the theoretical framework of our method by drift-diffusion simulations and demonstrate the applicability of ITCE to organic and perovskite semiconductor-based thin-film solar cells. Furthermore, we examine the field dependence of charge generation efficiency and find our ITCE results to be in excellent agreement with those obtained via time-delayed collection field measurements conducted on the same devices.

Dynamics of Excitons in Conjugated Molecules and Organic Semiconductor Systems

The exciton, an excited electron-hole pair bound by Coulomb attraction, plays a key role in photophysics of organic molecules and drives practically important phenomena such as photoinduced mechanical motions of a molecule, photochemical conversions, energy transfer, generation of free charge carriers, etc. Its behavior in extended π-conjugated molecules and disordered organic films is very different and very rich compared with exciton behavior in inorganic semiconductor crystals. Due to the high degree of variability of organic systems themselves, the exciton not only exerts changes on molecules that carry it but undergoes its own changes during all phases of its lifetime, that is, birth, conversion and transport, and decay. The goal of this review is to give a systematic and comprehensive view on exciton behavior in π-conjugated molecules and molecular assemblies at all phases of exciton evolution with emphasis on rates typical for this dynamic picture and various consequences of the above dynamics. To uncover the rich variety of exciton behavior, details of exciton formation, exciton transport, exciton energy conversion, direct and reverse intersystem crossing, and radiative and nonradiative decay are considered in different systems, where these processes lead to or are influenced by static and dynamic disorder, charge distribution symmetry breaking, photoinduced reactions, electron and proton transfer, structural rearrangements, exciton coupling with vibrations and intermediate particles, and exciton dissociation and annihilation as well.

Li-Doped ZnO Electron Transport Layer for Improved Performance and Photostability of Organic Solar Cells

Organic solar cells (OSCs) based on an inverted architecture generally have better stability compared to those based on a standard architecture. However, the photoactive area of the inverted solar cells increases under ultraviolet (UV) or solar illuminatiom because of the too-high conductivity of the UV-illuminated zinc oxide (ZnO) interlayer. This limits the potential of the inverted solar cells for industrial applications. Herein, lithium-doped ZnO (Li-ZnO) films are employed as the cathode interlayer to construct inverted OSCs. The incorporation of Li ions is found to reduce the lateral conductivity of the UV-treated ZnO films because of the presence of Li ions, preventing the high-quality-growth of ZnO nanocrystals. This addresses the problem of having too-high conductivity in the UV-treated ZnO layer, causing the increased photoactive area of inverted solar cells. The overall performance of the solar cell is shown to be higher after the incorporation of Li ions in the ZnO layer, mainly due to the increased fill factor (FF), originating from the reduced trap-assisted recombination losses. Finally, the inverted solar cells based on the Li-ZnO interlayer are demonstrated to have a much better long-term stability, as compared to those based on ZnO. This allows the ZnO-based interlayers to be used for the mass production of organic solar cell modules.

Photophysical Properties of BODIPY Derivatives for the Implementation of Organic Solar Cells: A Computational Approach

Solar cells based on organic compounds are a proven emergent alternative to conventional electrical energy generation. Here, we provide a computational study of power conversion efficiency optimization of boron dipyrromethene (BODIPY) derivatives by means of their associated open-circuit voltage, short-circuit density, and fill factor. In doing so, we compute for the derivatives' geometrical structures, energy levels of frontier molecular orbitals, absorption spectra, light collection efficiencies, and exciton binding energies via density functional theory (DFT) and time-dependent (TD)-DFT calculations. We fully characterize four D-π-A (BODIPY) molecular systems of high efficiency and improved J _sc that are well suited for integration into bulk heterojunction (BHJ) organic solar cells as electron-donor materials in the active layer. Our results are twofold: we found that molecular complexes with a structural isoxazoline ring exhibit a higher power conversion efficiency (PCE), a useful result for improving the BHJ current, and, on the other hand, by considering the molecular systems as electron-acceptor materials, with P3HT as the electron donor in the active layer, we found a high PCE compound favorability with a pyrrolidine ring in its structure, in contrast to the molecular systems built with an isoxazoline ring. The theoretical characterization of the electronic properties of the BODIPY derivatives provided here, computed with a combination of ab initio methods and quantum models, can be readily applied to other sets of molecular complexes to hierarchize optimal power conversion efficiency.

Photoinduced Charge Transfer and Recombination Dynamics in Star Nonfullerene Organic Solar Cells

Nonfullerene acceptors (NFAs) are regarded as star candidates for efficient organic solar cells with power conversion efficiency (PCE) over 18%. In contrast to the rapid development of NFA materials, however, the underlying excited-state dynamics which fundamentally govern the device performance remains unclear. In this Perspective, we discuss recent advances and provide our insights on photoinduced charge transfer and combination dynamics in NFA-based organic solar cells (OSCs), including the biphasic hole-transfer process and its correlation with morphology, the role of driving force and Marcus normal region behavior on interfacial hole-transfer properties, and charge recombination energy loss by NFA triplet formation. We also discuss our understanding of how to control the charge-transfer and recombination processes by phase morphology and molecular design to improve OSC performance. Finally, we suggest a few research directions, including the interfacial charge transfer and separation mechanism, the origin of low fill factor, and complex excited-state dynamics in multicomponent OSCs.

Mechanistic Insights into Visible-Light-Driven Dearomative Fluoroalkylation Mediated by an Electron Donor-Acceptor Complex

Electron donor-acceptor (EDA) complex photochemistry has become a burgeoning topic in the synthetic radical chemistry mediated by visible light; however, the theoretical insights into the reaction mechanisms are limited. Herein, accurate electronic structure calculations at the CASPT2//CASSCF/PCM level of theory were performed to investigate the paradigm example of EDA complex-enabled photoreaction for visible-light-driven dearomative perfluoroalkylation of β-naphthol. The excitation energy levels of the EDA complex are controlled by noncovalent interactions because the photoinduced intermolecular charge is enhanced when the noncovalent interaction becomes weaker, leading to the broad spectra ranging from UVA (<380 nm) to visible light (>500 nm). The competitiveness of the radical-radical coupling over the radical chain pathway is also regulated due to the tunable radical concentrations varying the excitation wavelength.

Narrowband organic photodetectors - towards miniaturized, spectroscopic sensing

Omnipresent quality monitoring in food products, blood-oxygen measurement in lightweight conformal wrist bands, or data-driven automated industrial production: Innovation in many fields is being empowered by sensor technology. Specifically, organic photodetectors (OPDs) promise great advances due to their beneficial properties and low-cost production. Recent research has led to rapid improvement in all performance parameters of OPDs, which are now on-par or better than their inorganic counterparts, such as silicon or indium gallium arsenide photodetectors, in several aspects. In particular, it is possible to directly design OPDs for specific wavelengths. This makes expensive and bulky optical filters obsolete and allows for miniature detector devices. In this review, recent progress of such narrowband OPDs is systematically summarized covering all aspects from narrow-photo-absorbing materials to device architecture engineering. The recent challenges for narrowband OPDs, like achieving high responsivity, low dark current, high response speed, and good dynamic range are carefully addressed. Finally, application demonstrations covering broadband and narrowband OPDs are discussed. Importantly, several exciting research perspectives, which will stimulate further research on organic-semiconductor-based photodetectors, are pointed out at the very end of this review.

Increasing donor-acceptor spacing for reduced voltage loss in organic solar cells

The high voltage losses ([Formula: see text]), originating from inevitable electron-phonon coupling in organic materials, limit the power conversion efficiency of organic solar cells to lower values than that of inorganic or perovskite solar cells. In this work, we demonstrate that this [Formula: see text] can in fact be suppressed by controlling the spacing between the donor (D) and the acceptor (A) materials (DA spacing). We show that in typical organic solar cells, the DA spacing is generally too small, being the origin of the too-fast non-radiative decay of charge carriers ([Formula: see text]), and it can be increased by engineering the non-conjugated groups, i.e., alkyl chain spacers in single component DA systems and side chains in high-efficiency bulk-heterojunction systems. Increasing DA spacing allows us to realize significantly reduced [Formula: see text] and improved device voltage. This points out a new research direction for breaking the performance bottleneck of organic solar cells.

Intramolecular Charge Transfer of 1-Aminoanthraquinone and Ultrafast Solvation Dynamics of Dimethylsulfoxide

The intramolecular charge transfer (ICT) of 1-aminoanthraquinone (AAQ) in the excited state strongly depends on its solvent properties, and the twisted geometry of its amino group has been recommended for the twisted ICT (TICT) state by recent theoretical works. We report the transient Raman spectra of AAQ in a dimethylsulfoxide (DMSO) solution by femtosecond stimulated Raman spectroscopy to provide clear experimental evidence for the TICT state of AAQ. The ultrafast (~110 fs) TICT dynamics of AAQ were observed from the major vibrational modes of AAQ including the ν_C-N + δ_CH and ν_C=O modes. The coherent oscillations in the vibrational bands of AAQ strongly coupled to the nuclear coordinate for the TICT process have been observed, which showed its anharmonic coupling to the low frequency out of the plane deformation modes. The vibrational mode of solvent DMSO, ν_S=O showed a decrease in intensity, especially in the hydrogen-bonded species of DMSO, which clearly shows that the solvation dynamics of DMSO, including hydrogen bonding, are crucial to understanding the reaction dynamics of AAQ with the ultrafast structural changes accompanying the TICT.

Reducing Non-Radiative Voltage Losses by Methylation of Push-Pull Molecular Donors in Organic Solar Cells

Organic solar cells are approaching power conversion efficiencies of other thin-film technologies. However, in order to become truly market competitive, the still substantial voltage losses need to be reduced. Here, the synthesis and characterization of four novel arylamine-based push-pull molecular donors was described, two of them exhibiting a methyl group at the para-position of the external phenyl ring of the arylamine block. Assessing the charge-transfer state properties and the effects of methylation on the open-circuit voltage of the device showed that devices based on methylated versions of the molecular donors exhibited reduced voltage losses due to decreased non-radiative recombination. Modelling suggested that methylation resulted in a tighter interaction between donor and acceptor molecules, turning into a larger oscillator strength to the charge-transfer states, thereby ensuing reduced non-radiative decay rates.

Temperature Dependence of Tryptophan Fluorescence Lifetime as an Indicator of Its Microenvironment Dynamics

The spectral-kinetic characteristics of the fluorescence of the tryptophan molecule in an aqueous solution and in the composition of a protein (albumin) were studied in the temperature range from -170 to 25°C. To explain the observed changes in the spectra and the tryptophan fluorescence lifetime with temperature, a model of transitions between the excited and ground states involving a charge-transfer state was used, which takes into account the nonlinear nature of the dynamics of these transitions. In these processes, an important role is played by the interaction of tryptophan molecules with its microenvironment, as well as rearrangements in the system of hydrogen bonds of the water-protein matrix surrounding the tryptophan molecule.

Influence of static disorder of charge transfer state on voltage loss in organic photovoltaics

Spectroscopic measurements of charge transfer (CT) states provide valuable insight into the voltage losses in organic photovoltaics (OPVs). Correct interpretation of CT-state spectra depends on knowledge of the underlying broadening mechanisms, and the relative importance of molecular vibrational broadening and variations in the CT-state energy (static disorder). Here, we present a physical model, that obeys the principle of detailed balance between photon absorption and emission, of the impact of CT-state static disorder on voltage losses in OPVs. We demonstrate that neglect of CT-state disorder in the analysis of spectra may lead to incorrect estimation of voltage losses in OPV devices. We show, using measurements of polymer:non-fullerene blends of different composition, how our model can be used to infer variations in CT-state energy distribution that result from variations in film microstructure. This work highlights the potential impact of static disorder on the characteristics of disordered organic blend devices.

Enhanced Charge Selectivity via Anodic-C60 Layer Reduces Nonradiative Losses in Organic Solar Cells

Interfacial layers in conjunction with suitable charge-transport layers can significantly improve the performance of optoelectronic devices by facilitating efficient charge carrier injection and extraction. This work uses a neat C₆₀ interlayer on the anode to experimentally reveal that surface recombination is a significant contributor to nonradiative recombination losses in organic solar cells. These losses are shown to proportionally increase with the extent of contact between donor molecules in the photoactive layer and a molybdenum oxide (MoO₃) hole extraction layer, proven by calculating voltage losses in low- and high-donor-content bulk heterojunction device architectures. Using a novel in-device determination of the built-in voltage, the suppression of surface recombination, due to the insertion of a thin anodic-C₆₀ interlayer on MoO₃, is attributed to an enhanced built-in potential. The increased built-in voltage reduces the presence of minority charge carriers at the electrodes-a new perspective on the principle of selective charge extraction layers. The benefit to device efficiency is limited by a critical interlayer thickness, which depends on the donor material in bilayer devices. Given the high popularity of MoO₃ as an efficient hole extraction and injection layer and the increasingly popular discussion on interfacial phenomena in organic optoelectronic devices, these findings are relevant to and address different branches of organic electronics, providing insights for future device design.

Charge Transfer Excitation and Asymmetric Energy Transfer at the Interface of Pentacene-Perfluoropentacene Heterostacks

High-performance solar cells demand efficient charge-carrier excitation, separation, and extraction. These requirements hold particularly true for molecular photovoltaics, where large exciton binding energies render charge separation challenging at their commonly complex donor-acceptor interface structure. Among others, charge-transfer (CT) states are considered to be important precursors for exciton dissociation and charge separation. However, the general nature of CT excitons and their formation pathways remain unclear. Layered quasiplanar crystalline molecular heterostructures of the prototypical donor-acceptor system pentacene-perfluoropentacene studied at cryogenic temperatures are a paramount model system to gain insights into the underlying physical mechanism. In particular, a detailed experiment-theory analysis on a layered heterojunction featuring perfluoropentacene in its π-stacked polymorph and pentacene in the Siegrist phase indicates that exciton diffusion in unitary films can influence the formation efficiency of CT excitons localized at internal interfaces for these conditions. The correlation of the structural characteristics, that is, the molecular arrangement at the interfaces, with their absorption and photoluminescence excitation spectra is consistent with exciton transfer from pentacene to the CT exciton state only, whereas no transfer of excitons from the perfluoropentacene is detected. Electronic structure calculations of the model systems and investigation of coupling matrix elements between the various electronic states involved suggest hampered exciton diffusion toward the internal interface in the perfluoropentacene films. The asymmetric energy landscape around an idealized internal donor-acceptor interface thus is identified as a reason for asymmetric energy transfer. Thus, long-range effects apparently can influence charge separation in crystalline molecular heterostructures, similar to band gap bowing, which is well established for inorganic pn-junctions.

Identifying the Molecular Origins of High-Performance in Organic Photodetectors Based on Highly Intermixed Bulk Heterojunction Blends

A bulk-heterojunction (BHJ) structure of organic semiconductor blend is widely used in photon-to-electron converting devices such as organic photodetectors (OPD) and photovoltaics (OPV). However, the impact of the molecular structure on the interfacial electronic states and optoelectronic properties of the constituent organic semiconductors is still unclear, limiting further development of these devices for commercialization. Herein, the critical role of donor molecular structure on OPD performance is identified in highly intermixed BHJ blends containing a small-molecule donor and C₆₀ acceptor. Blending introduces a twisted structure in the donor molecule and a strong coupling between donor and acceptor molecules. This results in ultrafast exciton separation (<1 ps), producing bound (binding energy ∼135 meV), localized (∼0.9 nm), and highly emissive interfacial charge transfer (CT) states. These interfacial CT states undergo efficient dissociation under an applied electric field, leading to highly efficient OPDs in reverse bias but poor OPVs. Further structural twisting and molecular-scale aggregation of the donor molecules occur in blends upon thermal annealing just above the transition temperature of 150 °C at which donor molecules start to reorganize themselves without any apparent macroscopic phase-segregation. These subtle structural changes lead to significant improvements in charge transport and OPD performance, yielding ultralow dark currents (∼10^-10 A cm^-2), 2-fold faster charge extraction (in μs), and nearly an order of magnitude increase in effective carrier mobility. Our results provide molecular insights into high-performance OPDs by identifying the role of subtle molecular structural changes on device performance and highlight key differences in the design of BHJ blends for OPD and OPV devices.

Effects on Photovoltaic Characteristics by Organic Bilayer- and Bulk-Heterojunctions: Energy Losses, Carrier Recombination and Generation

We investigate the photovoltaic characteristics of organic solar cells (OSCs) for two distinctly different nanostructures, by comparing the charge carrier dynamics for bilayer- and bulk-heterojunction OSCs. Most interestingly, both architectures exhibit fairly similar power conversion efficiencies (PCEs), reflecting a comparable critical domain size for charge generation and charge recombination. Although this is, at first hand, surprising, a detailed analysis points out the similarity between these two concepts. A bulk-heterojunction architecture arranges the charge generating domains in a 3D ensemble across the whole bulk, while bilayer architectures arrange the specific domains on top of each other, rather than sharp bilayers. Specifically, for the polymer PBDB-T-2F, we find that the enhanced charge generation in a bulk composite is partially compensated by reduced recombination in the bilayer architecture, when nonfullerene acceptors (NFAs) are used instead of a fullerene acceptor. Overall, we demonstrate that bilayer-heterojunction OSCs with NFAs can reach competitive PCEs compared to the corresponding bulk-heterojunction OSCs because of reduced nonradiative open-circuit voltage losses, and suppressed trap-assisted recombination, as a result of a vertically separated donor-to-acceptor nanostructure. In contrast, the bilayer-heterojunction OSCs with the fullerene acceptor exhibited poor photovoltaic characteristics compared to the corresponding bulk devices because of highly aggregated acceptor molecules on top of the polymer donor. Although free carrier generation is reduced in a in a bilayer-heterojunction, because of reduced donor/acceptor interfaces and a limited exciton diffusion length, more favorable transport pathways for unipolar charge collection can partially compensate the aforementioned disadvantages. We propose that the unique properties of NFAs may open a technical venue for the bilayer-heterojunction as a great and easy alternative to the bulk heterojunction.

Unraveling the influence of non-fullerene acceptor molecular packing on photovoltaic performance of organic solar cells

In non-fullerene organic solar cells, the long-range structure ordering induced by end-group π-π stacking of fused-ring non-fullerene acceptors is considered as the critical factor in realizing efficient charge transport and high power conversion efficiency. Here, we demonstrate that side-chain engineering of non-fullerene acceptors could drive the fused-ring backbone assembly from a π-π stacking mode to an intermixed packing mode, and to a non-stacking mode to refine its solid-state properties. Different from the above-mentioned understanding, we find that close atom contacts in a non-stacking mode can form efficient charge transport pathway through close side atom interactions. The intermixed solid-state packing motif in active layers could enable organic solar cells with superior efficiency and reduced non-radiative recombination loss compared with devices based on molecules with the classic end-group π-π stacking mode. Our observations open a new avenue in material design that endows better photovoltaic performance.

Organic-Inorganic Charge Transfer Complex with Charge Modulation after Electrical Pre-biasing

Several breakthroughs in organic optoelectronic devices with new applications and performance improvement have been made recently by exploiting novel properties of charge transfer complexes (CTCs). In this work, a CTC film formed by coevaporating molybdenum(VI) oxide and pentacene (MoO₃:pentacene) shows a strong dipole of 2.4 eV with direction controllability via pre-biasing with an external voltage. While CTCs are most widely known for their much red-shifted energy gaps, there is so far no report on their controllable dipoles. By controlling this dipole with an electrical pre-bias in a MoO₃:pentacene CTC based device, current changes over 2 orders of magnitude can be achieved. Kelvin probe force microscopy further confirms that surface potential of the MoO₃:pentacene film can be modulated by an external electric field. It is shown for the first time that a dipole of controllable direction can be set up inside a CTC layer by pre-biasing. This concept is further tested by incorporating the CTC layer in organic photovoltaic (OPV) devices. It was demonstrated that by pre-biasing the OPV devices in different directions, their open circuit voltages (V_oc) can be significantly tuned via the built-in potentials.

High Triplet Energy Host Materials for Blue TADF OLEDs-A Tool Box Approach

The synthesis of stable blue TADF emitters and the corresponding matrix materials is one of the biggest challenges in the development of novel OLED materials. We present six bipolar host materials based on triazine as an acceptor and two types of donors, namely, carbazole, and acridine. Using a tool box approach, the chemical structure of the materials is changed in a systematic way. Both the carbazole and acridine donor are connected to the triazine acceptor via a para- or a meta-linked phenyl ring or are linked directly to each other. The photophysics of the materials has been investigated in detail by absorption-, fluorescence-, and phosphorescence spectroscopy in solution. In addition, a number of DFT calculations have been made which result in a deeper understanding of the photophysics. The presence of a phenyl bridge between donor and acceptor cores leads to a considerable decrease of the triplet energy due to extension of the overlap electron and hole orbitals over the triazine-phenyl core of the molecule. This decrease is more pronounced for the para-phenylene than for the meta-phenylene linker. Only direct connection of the donor group to the triazine core provides a high energy of the triplet state of 2.97 eV for the carbazole derivative CTRZ and 3.07 eV for the acridine ATRZ. This is a major requirement for the use of the materials as a host for blue TADF emitters.

Revisiting the Charge-Transfer States at Pentacene/C60 Interfaces with the GW/Bethe-Salpeter Equation Approach

Molecular orientations and interfacial morphologies have critical effects on the electronic states of donor/acceptor interfaces and thus on the performance of organic photovoltaic devices. In this study, we explore the energy levels and charge-transfer states at the organic donor/acceptor interfaces on the basis of the fragment-based GW and Bethe-Salpeter equation approach. The face-on and edge-on orientations of pentacene/C60 bilayer heterojunctions have employed as model systems. GW+Bethe-Salpeter equation calculations were performed for the local interface structures in the face-on and edge-on bilayer heterojunctions, which contain approximately 2000 atoms. Calculated energy levels and charge-transfer state absorption spectra are in reasonable agreements with those obtained from experimental measurements. We found that the dependence of the energy levels on interfacial morphology is predominantly determined by the electrostatic contribution of polarization energy, while the effects of induction contribution in the edge-on interface are similar to those in the face-on. Moreover, the delocalized charge-transfer states contribute to the main absorption peak in the edge-on interface, while the face-on interface features relatively localized charge-transfer states in the main absorption peak. The impact of the interfacial morphologies on the polarization and charge delocalization effects is analyzed in detail.

Organic Photodetectors for Next-Generation Wearable Electronics

Next-generation wearable electronics will need to be mechanically flexible and stretchable such that they can be conformally attached onto the human body. Photodetectors that are available in today's market are based on rigid inorganic crystalline materials and they have limited mechanical flexibility. In contrast, photodetectors based on organic polymers and molecules have emerged as promising alternatives due to their inherent mechanical softness, ease of processing, tunable optoelectronic properties, good light sensing performance, and biocompatibility. Here, the recent advances of organic photodetectors in terms of both optoelectronic and mechanical properties are outlined and discussed, and their application in wearable electronics including health monitoring sensors, artificial vision, and self-powering integrated devices are highlighted.

Relation between the Electronic Properties of Regioregular Donor-Acceptor Terpolymers and Their Binary Copolymers

By analyzing the optical band gap and energy levels of seven different regioregular terpolymers in which two different electron-rich donor moieties are alternating with a common electron-deficient acceptor unit along the backbone, we establish a direct correlation with the properties of the corresponding binary copolymers in which one donor and one acceptor are combined. For this study, we use diketopyrrolopyrrole as the common acceptor and different π-conjugated aromatic oligomers as donors. We find that the optical band gap and frontier orbital energies of the terpolymers are the arithmetic average of those of the parent copolymers with remarkable accuracy. The same relationship is also found for the open-circuit voltage of the bulk heterojunction solar cells made with the ter- and copolymers in combination with [6,6]-phenyl-C₇₁-butyric acid methyl ester. Comparison of these findings with data in the literature suggests that this is a universal rule that can be used as a tool when designing new π-conjugated polymers. The experimental results are supported by a semiempirical quantum chemical model that accurately describes the energy levels of the terpolymers after parametrization on the energy levels of the copolymers and also provides a theoretical explanation for the observed arithmetic relations.

The Cost of Converting Excitons into Free Charge Carriers in Organic Solar Cells

Efficient exciton dissociation and subsequent generation of free charge carriers at the organic donor-acceptor interface requires a number of electron-transfer processes. It is a common view that these steps result in an unavoidable energy loss in organic photovoltaic devices that is not present in other types of solar cells. The currently best performing organic solar cells with power conversion efficiencies over 16% challenge this view, and no interfacial charge-transfer states with energy significantly lower than the strongly absorbing singlet states are detected within the gap of the used donor and acceptor materials. This Perspective will discuss implications, the remaining sources of energy loss, and open questions to be solved to achieve power conversion efficiencies over 20%.

Impact of the donor polymer on recombination via triplet excitons in a fullerene-free organic solar cell

The greater chemical tunability of non-fullerene acceptors enables fine-tuning of the donor-acceptor energy level offsets, a promising strategy towards increasing the open-circuit voltage in organic solar cells. Unfortunately, this approach could open an additional recombination channel for the charge-transfer (CT) state via a lower-lying donor or acceptor triplet level. In this work we investigate such electron and hole back-transfer mechanisms in fullerene-free solar cells incorporating the novel molecular acceptor 2,4-diCN-Ph-DTTzTz. The transition to the low-driving force regime is studied by comparing blends with well-established donor polymers P3HT and MDMO-PPV, which allows for variation of the energetic offsets at the donor-acceptor interface. Combining various optical spectroscopic techniques, the CT process and subsequent triplet formation are systematically investigated. Although both back-transfer mechanisms are found to be energetically feasible in both blends, markedly different triplet-mediated recombination processes are observed for the two systems. The kinetic suppression of electron back-transfer in the blend with P3HT suggests that energy losses due to triplet formation on the polymer can be avoided, regardless of favorable energetic alignment.

Eco-Compatible Solvent-Processed Organic Photovoltaic Cells with Over 16% Efficiency

Recent advances in nonfullerene acceptors (NFAs) have enabled the rapid increase in power conversion efficiencies (PCEs) of organic photovoltaic (OPV) cells. However, this progress is achieved using highly toxic solvents, which are not suitable for the scalable large-area processing method, becoming one of the biggest factors hindering the mass production and commercial applications of OPVs. Therefore, it is of great importance to get good eco-compatible processability when designing efficient OPV materials. Here, to achieve high efficiency and good processability of the NFAs in eco-compatible solvents, the flexible alkyl chains of the highly efficient NFA BTP-4F-8 (also known as Y6) are modified and BTP-4F-12 is synthesized. Combining with the polymer donor PBDB-TF, BTP-4F-12 shows the best PCE of 16.4%. Importantly, when the polymer donor PBDB-TF is replaced by T1 with better solubility, various eco-compatible solvents can be applied to fabricate OPV cells. Finally, over 14% efficiency is obtained with tetrahydrofuran (THF) as the processing solvent for 1.07 cm² OPV cells by the blade-coating method. These results indicate that the simple modification of the side chain can be used to tune the processability of active layer materials and thus make it more applicable for the mass production with environmentally benign solvents.

Local Excitation/Charge-Transfer Hybridization Simultaneously Promotes Charge Generation and Reduces Nonradiative Voltage Loss in Nonfullerene Organic Solar Cells

High power conversion efficiencies in state-of-the-art nonfullerene organic solar cells (NF OSCs) call for elucidation of the underlying working mechanisms of both high photocurrent densities and low nonradiative voltage losses under small energy offsets. Here, to address this fundamental issue, we have assessed the nature of interfacial charge-transfer (CT) states in a representative small-molecule NF OSC (DRTB-T:IT-4F) by time-dependent density functional theory calculations. The calculated results point to the fact that the CT states can borrow considerable oscillator strengths from the energy-close local excitation (LE) states or be fully hybridized with these LE states by molecular aggregation at the donor-acceptor interfaces. The LE/CT hybridization can promote charge generation by direct population of thermalized CT or LE/CT states under illumination. At the same time, the increased oscillator strengths of the lowest CT state will improve the luminescence quantum efficiencies and thus reduce nonradiative voltage losses. Our work suggests that it is crucial to tune the LE/CT hybridization by optimization of the donor and acceptor molecular and interfacial structures to further improve the NF OSC performance.

Emissive and charge-generating donor-acceptor interfaces for organic optoelectronics with low voltage losses

Intermolecular charge-transfer states at the interface between electron donating (D) and accepting (A) materials are crucial for the operation of organic solar cells but can also be exploited for organic light-emitting diodes^1,2. Non-radiative charge-transfer state decay is dominant in state-of-the-art D-A-based organic solar cells and is responsible for large voltage losses and relatively low power-conversion efficiencies as well as electroluminescence external quantum yields in the 0.01-0.0001% range^3,4. In contrast, the electroluminescence external quantum yield reaches up to 16% in D-A-based organic light-emitting diodes^5-7. Here, we show that proper control of charge-transfer state properties allows simultaneous occurrence of a high photovoltaic and emission quantum yield within a single, visible-light-emitting D-A system. This leads to ultralow-emission turn-on voltages as well as significantly reduced voltage losses upon solar illumination. These results unify the description of the electro-optical properties of charge-transfer states in organic optoelectronic devices and foster the use of organic D-A blends in energy conversion applications involving visible and ultraviolet photons^8-11.